Books

Is this an Android tablet, do you know? Like, an Amazon Kindle or something?

There are several different types of tablets; some, the ones from Apple, run iOS. Others run Android, like smartphones. And others run Windows. Different software packages for each.

EDIT: It sounds like losing-place-in-text is a thing with dyslexia, so there is some software that has been designed around it, but can't really know what to use without knowing the platform that you're on.

Like, this guy has some commercial website reader thing that talks about it:

https://readermode.io/

Probably not what you want, since you're looking for an ebook reader, but he talks about how having a highlighted line can avoid the problem and how some fonts can reduce the effect by have letter shapes that vary more, stuff like Comic Sans.

This comment says that the Android app for Kindle reader software already has such a feature. I'm not sure if you're using the Kindle app or not, but if so, you might already have that feature:

https://old.reddit.com/r/ereader/comments/18s3t65/highlight_function_for_reading/

I just checked it on a Kindle Paperwhite + the Windows Desktop app and an Onyx Boox Nova Air 2 (and a Boox Boox Note Air 3 C, a Boox Tab X, and a Samsung Smartphone): On the Paperwhite + desktop app I could not find it, but on the devices which use the Android app for Kindle there is something called "reading ruler" that highlights the line you are currently on (I am German so maybe this is not the exact translation you'll find). To access this, you just open a book, tap in the middle of the screen to open up the menu, go to the "Aa" menu and there to "More", and there (sadly not on the Paperwhite) you find the "reading ruler" (Leselineal).

This means you just need a device that runs the Kindle app for Android (Nova Air is on Android 11, NA3C has A12, Samsung has A13). Maybe your reader does that (I am not familiar with your Pocketbook, sorry).